Ah, they're just wannabe Aphyosemion killifish....
It's unfortunate we can't get precise collecting data on a lot of these fish, so those who care could start to put the puzzle together. The variation may be random genetics producing local forms, but it takes time for all that to happen, and it can tell anyone who cares about drainages, biogeography, natural history of river systems...
I know if I were a fisherman in the region and I caught a new population of pencilfish, I'd be sorely tempted to lie about where. It's a competitive business, and an advantage like that might get you the cash to send your kids to school. With killies, we've become used to seeing geographical set colour forms (temporarily set, as they can change). People can look at a C. biteniatum from around Lagos and recognize it compared to one from the Cross River or from Benin. If future DNA studies show them to be different species, the question will have been asked because of these variations, not in individuals but in populations.
I really enjoy seeing these different forms of Nannostomus. Where I live, you would never know of this diversity because you have to look very hard to find any of these fish.
